Microwave landing systems facilitate aircraft approach and landing operations. Such systems include so-called precision distance measuring equipment (DME/P) which provides aircraft distance information by measuring total round-trip time between pulse interrogations from an airborne transmitter and replies from a ground transponder. This type of system is capable of providing high accuracy ranging information in the severe multipath environment encountered during landing operations.
Range measurements in the airborne transmitter and replay delay timing in the ground transponder requires detection of the transmitted and received DME interrogation pulses. Accurate methods for estimating the time-of-arrival (TOA) of such pulses are thus critical to achieving useful ranging information. One such method for determining DME pulse time-of-arrival utilizes a so-called delay, attenuate and compare (DAC) detector, which functions to compare a delayed version of the DME pulse to an attenuated version thereof. A pulse time-of-arrival detector output is declared when the delayed signal exceeds the attenuated signal. This output is then used for timing purposes in the remainder of the decoding process.
Prior art microwave landing systems incorporating a DAC detector in the DME/P receiver provide reasonably adequate detection of the DME pulse. However, the DAC detector also produces time-of-arrival outputs due to noise generated in the DME/P receiver. Such outputs are undesirable because they adversely affect the timing in the remainder of the decoding process.
There is therefore a need to provide a method and apparatus for generating a timing signal for use in a time-of-arrival detection system which properly represents a pulse detection rather than receiver noise detections.